Development of multi-resonant thermally activated delayed fluorescence emitters and investigation of the impact of hydrogen bonding on the room temperature phosphorescence

Abstract

Thermally activated delayed fluorescence (TADF) materials stand out as a highly promising class of emitters for organic light-emitting diodes (OLEDs), capable of achieving up to 100% internal quantum efficiency. Most of these are organic and so are more sustainable to produce than the commercially employed phosphorescent organometallic complexes. Multiresonant TADF (MR-TADF) emitters have garnered significant attention, particularly as they are bright and generally show narrowband emission, making them well-suited for high-definition (HD) display applications. This work is centered on developing new designs for MR-TADF emitters that span the entire emission color spectrum for efficient OLEDs. The thesis also explores the impact of restricting molecular motion using hydrogen bonding to suppress nonradiative decay and thus turn on room temperature phosphorescence.

Chapter 1 presents the overview the mechanisms of photoluminescence and electroluminescence along with an overview of the development of TADF emitters, with a specific focus on the MR-TADF emitters.

Chapter 2 demonstrates how the character of the charge transfer excited state can be modulated from short-range charge transfer to long-rang charge transfer through attaching different numbers of donors with different electron-donating strengths onto a MR-TADF emitter core, DiKTa.

Chapter 3 documents how decorating the DiKTa core with three donors can lead to efficient green (3TPA-DiKTa) and red (3DPA-DiKTa) MR-TADF emitters. The devices with much supressed efficiency roll-off were obtained with aid of the HF-OLEDs stack.

In Chapter 4, we introduce a fluorene-bridged double carbonyl/amine-based MR-TADF emitter DDiKTa-F, formed by locking the conformation of the previously reported compound DDiKTa. Using this strategy, DDiKTa-F exhibits a narrower, brighter, and red-shifted emission.

Chapter 5 discusses the development of an orange MR-TADF emitter, DDiKTa-A, based on a design that bridges two DiKTa units via a central aniline.

Chapter 6 reveals the design of a blue MR-TADF emitter (DOBDiKTa) formed by the fusion of two MR-TADF emitters, DiKTa and tBuDOBNA, together. Using this strategy, this compound emits desirably at an intermediate blue emission between the sky blue of DiKTa and the purple of tBuDOBNA.

In Chapter 7, two MR-TADF emitters, 2GtBuCzCO2HDCzB and tBuCzCO2HDCzB, are presented. These emitters exhibited resistance to aggregation and aggregation-caused quenching, even in neat films, by encapsulating the MR-TADF emissive core DtBuCzB within flanking donor-acceptor TADF groups.

Chapter 8 demonstrated the generality of embedding a hydrogen-bonding guest fluorophore into a hydrogen bonded network as a means of activating the RTP through the restriction of the molecular motion.

Chapter 9 presented the experimental methods used in this thesis.